1. Field of the Invention
The present invention relates to a vacuum cleaner, and more particularly, to a vacuum cleaner that creates substantially less noise by using a vacuum pump with a lobed chamber.
2. Description of Related Technology
Although vacuum cleaners have become virtually indispensable, the noise they create limits their utility because other nearby activities often must cease during vacuuming.
There have been many approaches to reducing the environmental noise from vacuum cleaners. One rather obvious one is to incorporate sound insulating material in the vacuum cleaner housing. While this approach will somewhat reduce the noise level around the vacuum cleaner, it does not actually attack at its source any of the noise generated by the vacuum cleaner. Another involves using muffler arrangements for the exhaust air flow. A more sophisticated approach to reducing exhaust noise uses a noise detector in the vacuum cleaner exhaust to provide a signal used to generate noise-canceling sound. A sampling of such approaches can be found in U.S. Pat. Nos. 4,418,443, 4,435,877, 4,532,713, 4,970,753, 5,502,869, 5,159,738, 5,499,423 and 5,513,417.
However, none of those approaches attacks two appreciable sources of noise in a vacuum cleaner. One of those sources is the high flow velocities that must be generated by existing vacuum cleaners to obtain a mass flow rate that will provide effective cleaning. The other is noise caused by the vacuum cleaner's rotating components.
According to well known principles, so-called "dipole noise," N.sub.db, caused by rotating components satisfies the relationship: EQU N.sub.db .varies..omega..sup.6 (1)
From equation (1) it can seen that dipole noise is proportional to the sixth power of the rotational speed .omega. of the flow-generating components of a vacuum cleaner. Therefore, very small increases or decreases in the rotational speed .omega. will have a great effect on the dipole noise.
The prior art approaches discussed above operate to mask the "jet noise" associated with the air stream exiting the vacuum cleaner housing. The approaches that use muffler arrangements generally seek to reduce the velocity of the air stream before allowing it to exit the vacuum cleaner. That approach results in meaningful jet noise reduction because jet noise scales to the eighth power of air flow velocity (that is, U.sup.8). Even further noise reductions would be possible if the velocity of the air flow exiting the vacuum cleaner impeller device were reduced.
The present invention uses a positive displacement vacuum pump to reduce noise, and there are no known vacuum cleaners that incorporate such a pump to create the pressure drop that produces the debris-entraining air flow in a vacuum cleaner. The reason for that lack in the prior art is quite likely due to the mechanical complexity of the most common types of positive displacement pumps. For example, a pump having a reciprocating piston would require complicated valving and parts manufactured to close tolerances. The cost of a vacuum cleaner incorporating such a pump would probably be much more than could be charged for a consumer product, and it would be far less reliable than existing vacuum cleaners that simply use a rotating impeller.
As a result, there are no known vacuum cleaners with a Wankel-type positive displacement pump. Wankel-type devices were simply a curiosity until solution of the problem of providing adequate sealing between the rotating "piston" and the walls of the stationary "cylinder." While solutions to these problems are now well known, they would probably be considered exotic for a product such as a vacuum cleaner. In any event, they would certainly drive up the cost of a vacuum cleaner and would require frequent replacement because the compressor in a vacuum cleaner is subject to abrasion from the particulate matter entrained in the air flow.